First of all, you can book the dining plan if you want it. You would need to ask for a ticketless room/dining package for annual passholders. You can also apply an AP room discount to the package. However, it will be a package reservation, not a room only, which means you have to pay it off 45 days before arrival, not when you check in. The deposit, cancellation and change rules for packages will also apply - they are quite different from those for room-only reservations.
As for savings on the dining plan, it entirely depends on how you use it. Used to be Disney's advertising would say the dining plan could save you up to 40% (the key words there are "up to" because the true savings depend on where you eat and what you order). Now, Disney's advertising states the dining plan can save up to 20%. I understand the phone agents are hard-selling the dining plan, but it's kind of misleading for them to say the dining plan will save you 40% in all instances.
What you need to do is look at how the dining plan can be used, then compare it with the way you usually eat on vacation. Are you aware there are three different dining plans? There is a sticky thread on the Dining Plan sub-board with the information about what they are and how they work. If you want the most popular plan, for which you receive one TS credit and one CS credit per night plus a snack, and you don't always order entree and dessert or you're not really into snacks, you may be better off with the TIW. There are a number of restaurants at Downtown Disney and the Swan/Dolphin that accept TIW and don't accept
DDP. TIW, however, doesn't apply to most counter service restaurants or to any snack/drink kiosks. All of this and more should be taken into consideration. None of the dining plans include alcoholic beverages (except at the dinner shows) so if you like wine or a cocktail with meals or at the lounges, consider that too, as TIW gives you a discount on those.